Underground coal gasification (UCG or ISC) is a process by which a coal seam is converted into a product gas, by combusting and gasifying the coal in-situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or syngas and can be used as a feedstock for various applications, including fuels production, chemicals production and power generation. The underground coal gasification technology is suitable for most coal reserves. Due to the environmental requirements for mining industry becoming increasingly strict, and considering relevant labour cost and construction cost, the technology is undoubtedly very attractive.
No matter whether the coal gasification process is conducted in-situ or on the surface, it is a series of chemical reactions converting coal into gas products. Generally, the chemical reactions involved are as following, wherein H2O and CO2 are the main gasification agents and O2 is the main oxidant:
C+O2→CO2 (Complete oxidation reaction)
CO+½)2→CO (Partial oxidation reaction)
C+H2O→H2+CO (Steam gasification reaction)
C+2H2→CH4 (Hydrogen gasification reaction)
C+CO2→2CO (Carbon dioxide gasification reaction)
CO+H2O⇔H2+CO2 (Water gas shift reaction)
CO+3H2⇔CH4H2O (Methanation reaction)
During the underground coal gasification process, a sub-surface completed well system is generally set up in the coal seam. The above mentioned completed well system includes injection well for injecting a variety of agents such as oxidant, gasification agent and coolant etc., a production well for removing product gas and other support wells, wherein casing and/or well liner are generally inserted in the injection well, production well and other support wells to connect each other, wherein the above mentioned support wells generally include the ignition well, syngas quench well, guard well, etc., and wherein, the injection well is generally a horizontal directional well while the production well and support well can generally be either horizontal directional wells or vertical wells.
Therefore, during the underground coal gasification process, the basic or most simple completed well system consists of an injection well and a production well, linked and provisioned with a casing and/or well liner. This is typically referred to as an underground coal gasifier unit or well pair.
During the underground coal gasification process, the relevant sub-surface zones include the combustion zone, gasification zone and pyrolysis zone, wherein: the combustion zone is in close proximity to the oxidant and gasification agent injection point, and coal is combusted and gasified in the presence of oxidant in the combustion zone; the gasification zone is located downstream of the combustion zone or radically around combustion zone, and coal is gasified and partially oxidized to produce product gas in the gasification zone; the pyrolysis zone is located downstream of the gasification zone, and pyrolysis reactions of coal occurs in the pyrolysis zone. The pyrolysis reactions of coal are generally not expected for a well-controlled underground coal gasification process. As coal is consumed or gasified, an underground coal gasification cavity within the coal seam develops and grows in size. This represents a gradual progression of the underground coal gasification process, until sub-surface coal reserve is completely consumed, leaving ash inside the coal seam.
During the underground coal gasification process, the produced product gas is referred to as raw syngas, which usually includes CO, CO2, H2, CH4 and solid particles, water, coal tar and hydrocarbons, and small amount of H2S, NH4 and COS etc. The actual composition of the above-mentioned product gas is dependent on multiple factors, including the oxidant used air, oxygen-enriched air, or pure oxygen), presence of water (coal seam water or ingress water from surrounding strata into the coal seam), coal quality and process parameters used (temperature and pressure etc.).
In the underground coal gasification process, it is generally preferred to use an oxidant with a higher oxygen concentration, since it can produce a better-quality product gas, specifically a higher syngas calorific value. However, if the oxygen concentration is high, such as above 35 vol %, it is necessary to use a coolant at the same time to reduce the risks of the high temperature in the combustion zone, the high burn-back rate and damage to the injection well liner. Therefore, it is beneficial to safely use high-concentration oxidants such as pure oxygen for the underground coal gasification process.
In addition, in present art, continuous injection of oxidant generally cannot be realized during the underground coal gasification process. In particular, when it is necessary to change the oxidant injection position during the underground coal gasification process, the oxidant injection is usually stopped. For example, for the underground coal gasification process utilising the retraction method, the oxidant injection point is generally required to be retracted periodically. As an example, the oxidant injection point is required to be retracted by 50 m after a 30 days' continuous gasification period, wherein the retraction process generally requires termination of oxidant injection. In this situation, the underground coal gasification process cannot be considered to be “continuous”, and such “discontinuity” could cause fluctuations of product gas quality, wherein large syngas property fluctuations can lead to operational challenges to downstream syngas processing applications. Therefore, it is beneficial if the real continuous oxidant injection operation can be implemented for the underground coal gasification process.
WO2014/043747 A1 published a device and method for carrying out an oxygen-enriched underground coal gasification process. It provides detailed oxygen injection device and method, wherein a specially designed oxygen lance is used to inject oxidant into an underground coal seam, and the mentioned oxygen lance comprises: a lance body having an internal pathway with a check valve inserted therein; a coiled tubing adapter connected to the rear end of the lance body, including a hole for inserting a thermocouple cable; at least one spacer tube connected to the forward end of the lance body; an injection nozzle connected to the forward end of the spacer tube; and a thermocouple for monitoring the temperature of the injection nozzle. Although the art mentioned the oxygen-enriched underground coal gasification process, the actual operation is the air blown process.
WO2014/186823 A1 published an oxidant and water injection device and method for underground coal gasification, wherein the device includes an oxidant conduit and a casing seal. The above-mentioned oxidant conduit has at least one opening at a downhole end and an upper end for injecting oxidant into the underground coal gasifier, where the upper open end is adapted for hydraulically connecting the flow conduit to the coiled tubing conduit; and the above mentioned casing seal is adapted for sealing the annulus between the oxidant conduit and the casing of a cased wellbore, where the casing seal has one or more pathways for injecting water into the underground coal gasifier. Although the oxidant can be substantially pure oxygen, the controlled retraction injection point (CRIP) method is applied, which corresponds to the underground coal gasification process in current art, wherein the process is separated by multiple retraction phases.
In summary, in the prior art, the underground coal gasification process still needs to be improved in some aspects, including: how to better control the combustion and gasification within the coal seam to provide high quality product syngas; how to achieve moving or retraction of oxidant injection device without interrupting oxidant injection; how to prevent the high downhole temperature to damage the oxidant injection device and fuse to the well liner in the horizontal wellbore; and how to better conduct subsurface monitoring to control the underground coal gasification process.
Therefore, further improvements are required to the current technology for underground coal gasification, especially to achieve continuous oxidant injection operation and the safe use of highly concentrated oxidant such as pure oxygen.